1. Field of the Invention
The present invention relates to hybrid silsesquioxane polymers suitable as low-k dielectrics in integrated circuits (IC's) and for other similar applications. In particular, the invention concerns a composition and processing method for thin films comprising polymer compositions of organic-inorganic hybrid materials.
2. Description of Related Art
Built on semiconducting substrates, integrated circuits comprise millions of transistors and other devices, which communicate electrically with one another and with outside packaging materials through multiple levels of vertical and horizontal wiring embedded in a dielectric material. Within the multilayer metallization structure, “vias” make up the vertical wiring, whereas “interconnects” form the horizontal wiring. Fabricating the metallization can involve the successive depositing and patterning of multiple layers of dielectric and metal to achieve electrical connection among transistors and to outside packaging material. The patterning for a given layer is often performed by a multi-step process comprising layer deposition, photoresist spin, photoresist exposure, photoresist develop, layer etch, and photoresist removal on a substrate. Alternatively, the metal may sometimes be patterned by first etching patterns into a layer of a dielectric material, filling the pattern with metal, then subsequently chemically/mechanically polishing the metal so that the metal remains embedded only in the openings of the dielectric. As an interconnect material, aluminum has been utilized for many years due to its high conductivity, good adhesion to SiO2, known processing methods (sputtering and etching) and low cost. Aluminum alloys have also been developed over the years to improve the melting point, diffusion, electromigration and other qualities as compared to pure aluminum. Spanning successive layers of aluminum, tungsten has traditionally served as the conductive via plug material.
In IC's, silicon dioxide, having a dielectric constant of around 4.0, has been the dielectric of choice, used in conjunction with aluminum-based and tungsten-based interconnects and via for many years.
The drive to faster microprocessors and more powerful electronic devices in recent years has resulted in very high circuit densities and faster operating speeds which—in turn—have required that higher conductivity metals and lower-k dielectrics (preferably below 3.0, more preferably below 2.5 dielectric constant) be used. In the past few years, VLSI (and ULSI) processes have been moving to copper damascene processes, where copper (or a copper alloy) is used for the higher conductance in the conductor lines and a spin-on or CVD process is used for producing low-k dielectrics which can be employed for the insulating material surrounding the conductor lines. To circumvent problems with etching, copper along with a barrier metal is blanket deposited over recessed dielectric structures consisting of interconnect and via openings and subsequently polished in a processing method known as the “dual damascene.” The bottom of the via opening is usually the top of an interconnect from the previous metal layer or, in some instances, the contacting layer to the substrate.
In addition to being lithographically patternable, the dielectric IC material should be easy to deposit or form, preferably at a high deposition rate and at a relatively low temperature. Once the material has been deposited or formed, it should also be readily patterned, and preferably patterned with small feature sizes if needed. The patterned material should preferably have low surface and/or sidewall roughness. It might also be desirable that such materials be hydrophobic to limit uptake of moisture (or other fluids), and be stable with a relatively high glass transition temperature (not degrade or otherwise physically and/or chemically change upon further processing or when in use).
Summarizing: aside from possessing a low dielectric constant, the ideal dielectric should have the following properties:    1. High modulus and hardness in order to bind the maze of metal interconnects and vias together as well as abet chemical mechanical polishing processing steps.    2. Low thermal expansion, typically less than or equal to that of metal interconnects.    3. Excellent thermal stability, generally in excess of 400° C.    4. No cracking, excellent fill and planarization properties    5. Excellent adhesion to dielectric, semiconductor, and metal materials.    6. Sufficient thermal conductivity to dissipate joule heating from interconnects and vias.    7. Material density that precludes absorption of solvents, moisture, or reactive gasses.    8. Allows desired etch profiles at very small dimensions.    9. Low current leakage, high breakdown voltages, and low loss-tangents.    10. Stable interfaces between the dielectric and contacting materials.
By necessity, low-k materials are usually engineered on the basis of compromises. Silicate-based low-k materials can demonstrate exceptional thermal stability and usable modulus but can be plagued by brittleness and cracking. Organic materials; by contrast, often show improved material toughness, but at the expense of increased softness, lower thermal stability, and higher thermal expansion coefficients.
Porous materials sacrifice mechanical properties and possess a strong tendency of absorbing chemicals used in semiconductor fabrication leading to reliability failures. Furthermore, these porous materials are mesoporous or micro porous with pore diameters in excess of 2 nm and pore volumes greater than 30%. Fluorinated materials can induce corrosion of metal interconnects, rendering a chip inoperative. Generally, the mechanical robustness and thermal conductivity of low-k materials is lower than the corresponding properties of their pure silicon dioxide analogues, making integration into the fabrication flow very challenging.
Further, known materials comprising exclusively inorganic bonds making up the siloxane matrix are brittle and have poor elasticity at high temperatures.
Organosiloxane materials are typically deposited via spin-on processing, however Chemical Vapor Deposition (CVD) is also a viable technique for the deposition of these materials. For example, published International Patent Application No. WO03/015129 discloses organosilicone low-k dielectric precursors, which are useful for producing porous, low-k dielectric, SiOC thin films, wherein the organosilicon precursor comprises at least one cleavable, organic functional group that, upon activation, rearranges, decomposes and/or is cleaved-off as a highly volatile liquid and/or gaseous by-product. Other organosilicone precursors comprising Si—O—C-in-ring cyclic siloxane compounds for use as precursors for forming insulator films by CVD are described in U.S. Pat. No. 6,440,876. When these siloxane precursors are contacted with the surface of a semiconductor or integrated circuit, they will react with the wafer surface forming a dielectric film. By ring-opening polymerization of these cyclic compounds, a dielectric film or layer will be formed.
U.S. Pat. No. 6,242,339 discloses an interconnection structure, in which a phenyl group, bonded to a silicon atom, is introduced into the silicon dioxide of the organic-containing silicon dioxide to produce a material suitable as an interlevel insulating film. Such a film can be processed just as easily as a conventional CVD oxide film; it has a relative dielectric constant as low as that of a hydrogen silsesquioxane (HSQ) film, and can adhere strongly to an organic film, an oxide film or a metal film. According to the patent, the number of devices that can be integrated within a single semiconductor integrated circuit can be increased without modification of the conventional semiconductor device manufacturing process to provide a high-performance semiconductor integrated circuit, operative at high speed and with lower power dissipation.
On the other hand, there are several examples of organosiloxane low-k materials made by spin-on deposition techniques. Spin-on low-k materials known in the prior art are mainly based on methyl- or phenyl-substituted organosiloxanes and combinations thereof. There are also some examples of adamantyl-substituted organosiloxanes.
The use of these types of polymers results, however, in fundamental problem due to their polarizability nature. The methyl-based siloxanes (also known as silsesquioxanes) will give relatively low electronic dielectric constant (polarizability), more specifically approximately 1.88, but—by contrast—their orientational dielectric constant (polarizability) is high, i.e., approximately 0.7, which significantly increases the total dielectric constant measured at low frequencies (10 kHz-10 GHz). Again, phenyl-based organosiloxanes have higher electronic dielectric constant (polarizability), i.e., approximately 2.4, but their orientational dielectric constant (polarizability) is lower (approximately 0.4) due to a relatively lower content of permanent dipoles in the material matrix. Therefore, the total dielectric constant cannot reach values close to 2.5 or lower, when ionic dielectric constant (polarizability) of approximately 0.15-0.3 is included, without the introduction of porosity (air having a dielectric constant of 1) into the film matrix. Moreover, the porous materials that are currently reaching a dielectric constant of less than 2.5 are typically highly porous, which makes the integration of such materials into the semiconductor device very difficult.
A particular problem with the adamantyl-substituted siloxanes known in the art is that they have been produced from alkoxy-precursors, which leave alkoxide residues in the matrix of the material. Such residues greatly impair the use and properties of the materials in particular as regards their dielectric properties. If residual alkoxides remain in the matrix, they tend to react over time and change materials properties by forming contaminating alcohol and water into the matrix. Such oxygenates decrease dielectric and leakage current behavior of the material. In addition, residual alkoxides, such as ethoxide-based materials, cause a dangling bond effect that causes higher leakage current for the material. Further, there are no industrially viable processes for producing suitable adamanlyl-substituted siloxane precursors.
Thus, the prior art contains no examples of dielectric materials for semi-conductor manufacture, which have desired properties of low dielectric constant with low controlled micro porosity, high thermal stability, and low cost. Further, there is a need for new precursors for hybrid organo-silsequioxane polymers.